I assume they is being used here as a gender-neutral singular pronoun, since the use of life rather than lives indicates there’s only one person involved.
The verb tenses are a bit of a mess. Change have always know (which is wrong under any circumstances — should be known rather than know) to always knew. (You don’t need to keep using had over and over again, and it’s actually better if you don’t. Once you establish, ‘This happened before the rest of the past-tense-narrated story,’ you can leave it alone.)
They had spent all their life trying to prevent the prophecy from coming true. They always knew when they would destroy Earth, but when the day finally came, they were glad to do it.